Fractal structures of kaolin and humic flcos by the mass-size method based on the microbalance and free-settling method based on particle image velocimetry technique were investigated. The results showed that kaolin and humic flocs were characterized by fractal structure. The three-dimensional fractal dimensions of Fe flocs with more compact structure ranged from 2.14 to 2.28 are larger than those of alum flocs ranged from 1.75 to 1.83 by the mass-size method. The fractal dimensions of humic flocs are smaller than those of kaolin flocs. The fractal dimension has a close relationship with the settling velocity of flocs. The larger the fractal dimension, the higher the settling velocity. The fractal dimensions by the mass-size method are in good agreement with those by the free-settling method. The measured settling velocities of fractal aggregates do not follow with Stokes' law.